chaging gallery image max-height to 90% of viewport

90vh = 90% of viewport height. 650px is the fallback for [browsers that do not support the vh unit](http://caniuse.com/#feat=viewport-units). assuming for the most part that older browsers are likely to be running on older computers and therefore lower resolution monitors. 650px seems appropriate for a 768px height display.
use `mediator` for these - https://github.com/guardian/frontend/blob/master/common/app/assets/javascripts/bootstraps/common.js#L309